Work is ongoing in 3GPP, 3rd Generation Partnership Project, and with some operators to specify and investigate the possibility to provide home and/or small area coverage for a limited number of users using a small base station, an Access Point Base Station, commonly called a Femto NodeB for WCDMA, Wideband Code Division Multiple Access, or Femto eNB (E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) NodeB) for LTE, Long Term Evolution. Other common names are HNB (Home NodeB) for WCDMA or HeNB (Home eNodeB) for LTE.
This Femto node would provide normal LTE/WCDMA coverage for the end users, a so called femto cell, and would be connected to the network using some kind of IP based transmission. One example is to use fixed broadband access (e.g. xDSL or Cable) to connect the Femto node to the network.
There are several ways to use the available frequencies between the femto layer and the traditional cellular deployment layer (in this document referred to as ‘macro’ layer even though it may comprise both macro, micro and pico cells). There are three Channel Deployment Scenarios (CDS) that are relevant, namely:                CDS1: One frequency used both by the Femto NodeBs and the Macro NodeBs. This CDS has some severe interference problems that will make it hard to work.        CDS2: One dedicated frequency used only by the Femto NodeBs and (at least) another frequency used only by the Macro NodeBs. This CDS could be the preferred one if only technical arguments are taken into account. However, it is very unlikely that operators are willing to dedicate whole frequencies for the femto layer, mostly due to economical reasons.        CDS3: One frequency used both by the Femto NodeBs and the Macro NodeBs and (at least) another frequency used only by the Macro NodeBs.        
CDS3 is expected to be a common way for deployments. However, if frequencies are used according to CDS3, interference will occur in certain scenarios.
A small number of Primary Scrambling Codes (PSC) may be allocated to the femto layer, e.g. ten codes. These PSCs are configured in a cell neighbor lists on the macro layer, and signaled to macro layer user equipments UEs for their idle mode cell selection and as potential HO candidates that should be monitored and measured. One of these PSC's is automatically assigned to each Femto NodeB at start up.
In a normal, macro-only scenario (that is, a network without femto cells), the serving RNC controlling the Macro UE would try to perform soft handover, that is, to add a leg to the new base station controlling the detected cell. However, soft handover between macrocells and femtocells is difficult to support and therefore unlikely to be available. The difficulties are related to the expected large number of femto cells and the limitation of space in neighbor lists which will make it hard to address a target femto cell. Furthermore a femto cell is expected to have restrictions with regards to allowed users, that is, have access control.
Therefore, when a Macro UE wants to perform a soft handover from a macro to a femto cell substantial interference may occur. Depending on the location of the Macro UE, this may lead to very poor performance of the interfered femto Node B.